A flame arrestor apparatus usually comprises flame extinguishing elements which have very small diameter, typically less than 0.040-inch diameter channels that permit gas flow but prevent flame transmission by quenching or extinguishing combustion. This results from the transfer of heat (enthalpy) from the flame (high temperature) to the solid matrix of channels (low temperature) which effectively provide a substantial heat sink.
The quenching process is based on the drastic temperature difference between the flame and channel matrix material. As such, this is a transport process that not only depends on the temperature gradient, but also on the channel hydraulic diameter and the thermal conduction (diffusivity) properties of the gas.
The rate of heat loss from the flame is significantly affected by the level of turbulence within the flame arrestor channel. The turbulence is associated with the flow of unburnt gas through the flame arrestor as instigated by the pressure rise that accompanies a flame front to the element. The flame induced flow is always in the same direction as the impinging flame travel. The pressure rise can range from a small fraction to more than 100 times the initial (pre-ignition) absolute pressure in the system.
Two of the most common types of flame arrestor elements are the crimped ribbon type such as described in U.S. Pat. No. 4,909,730 and the parallel plate type as described in Canadian patent 1,057,187. The major advantage of these constructions is that it is possible to build a device with a fairly large percentage of open flow area per unit cross section while maintaining precise channel dimensions. This is very important because flame arrestors are often used in installations where large volumes of gas must be vented with minimal back pressure on the system. It is generally understood that even small deviations in channel dimensions can compromise flame arrestor performance. These can be referred to as straight path flame arrestors because the gas flow takes a straight path from the channel entrance to the exit.
A major disadvantage of the straight path units is that they do not extract heat from the flame very efficiently. One method commonly used by designers to overcome the low heat transfer efficiency of straight path units is to further reduce the hydraulic diameter of the straight path channels. This is intended to increase heat transfer efficiency by increasing the lateral area of heat loss per unit volume of flame front. However, the diameter reduction further increases the tendency for laminar flow which in turn further reduces heat transfer. The channels of reduced diameter also become clogged and fouled by liquids or particles that are usually present in the system.
Another method frequently used to overcome the low heat transfer efficiency of straight channels is to design an element consisting entirely of tortuous path channels. Examples of these include stacked expanded metal or wire mesh, sintered metal or ceramic, packed beads, and steel wood plug. The disadvantages of tortuous path elements is that they clog readily, are difficult to clean, and they have unacceptably high flow pressure drops, resulting in the need for excessively large element flow cross sections.
There are several examples of these types of systems, as given in prior patents. Examples of wire mesh systems are described in U.S. Pat. No. 1,701,805 and Canadian patent 666,952. The wire mesh element comprises a plurality of layers which function as a flame flow interrupter. Other systems which develop tortuous paths for the gas flow are described in Canadian patents 565,942 and 709,337. Such tortuous paths are provided by beads, particles, and the like which are also used in a system described in U.S. Pat. No. 2,044,573.
A system which involves crimped ribbon and laminar flow channels for the flame arrestor element are described in U.S. Pat. Nos. 2,087,170; 2,789,238; and 3,287,094.
Other types of systems involve nesting of plates, such as described in U.S. Pat. Nos. 1,826,487; 1,960,043; 2,068,421; 2,186,752; 2,618,539; 2,758,018; and 3,903,646. In these flame arrestors the plates are nested in a manner to provide flame extinguishing properties by transfer of heat from the flame front to the flame arrestor element.
As noted, the difficulty with these flame arrestor systems is that the channels through which the flame front flows cause a laminar flow in the flame front. This is detrimental from the standpoint of arresting high pressure flames, particularly detonations. It has been found however that in order to ensure extinction of the detonation type of flame, additional modifications must be made to the arrestor having a wire mesh plate or crimped metal design. In U.S. Pat. No. 4,909,730 a detonation attenuating device is positioned upstream of the flame quenching elements. Testing has demonstrated that the presence of the cup-shaped detonation attenuating device to attenuate impinging shock waves due to detonation significantly improves the overall performance of the flame arrestor having the standard type of crimped metal heat transfer flame arresting elements. However, the use of the detonation attenuator cup causes significant flow restrictions in the gas exhausting system and complicates manufacture of the device.
Although systems have been provided which can arrest flame fronts of the deflagration or detonation type, such systems require the use of element designs which develop significant back pressure. Designs which enhance heat transfer of the flame front to the arresting element have the overriding flow limiting factor of small diameter channels which induce laminar flow, and hence reduce the effectiveness of heat transfer from the flame front to the elements due to the boundary layer effect of the laminar flow through the small diameter channels of the element. There therefore continues to be a need for a flame arresting device which can extinguish all types of travelling flames ranging from deflagration types through to the very high pressure detonation types without overly restricting normal gas flows through the arrestor.